An endoscope is often used for viewing areas which are difficult to access, for example body cavities. In order to evaluate and document the state of an object viewed, a camera may be attached externally to the endoscope. If the camera is photographic said state can be photographically recorded. If a light converter element is used, for example a CCD element for converting optical signals into electrical video signals, records can be made by means of a video recorder. In order to produce images allowing precise diagnosis, the illumination of said object must be optimum. If a constant amount of light is delivered by the light source, said object may reflect the light to an undesirable extent, or may itself be under illuminated.
DE-B-3l 18 341 discloses a light regulating system for an endoscope in which the image of the object is recorded by means of a television camera. The amount of light which impinges on the light conductor is regulated so that the voltage of the video signal is kept at an essentially constant voltage level regardless of any changes in the distance between the object viewed and the distal end of the endoscope.
Since the brightness of the image is regulated with the aid of the mean value of the video signal, the object viewed is not always correctly illuminated.
DE-A-37 43 090 discloses an illumination regulating system for an endoscope in which the illuminating light and the brightness of the object viewed are regulated automatically, regardless of the size of the image which can be transmitted by means of the image transmitter. The change in the level of the video signal as a result of using endoscopes having different image circle diameters and, therefore, different image ranges is compensated for in that width of the endoscopic image is scanned line by line, the greatest width being stored as the voltage level by a peak value holding circuit and being added onto the actual value of the video signal. The regulation of the video signal is in accordance with the greatest width, rather than the area of the image.
DE-A-35 09 825 discloses an illumination regulating system for a light projector, enabling an object to be illuminated correctly at its distance from the distal end of the endoscope and the amount of light to be limited to a maximum value. A theoretical-actual value comparison ensures that the video signal keeps within the maximum permitted amplitude. Although this regulating system prevents swamping out of the brightest parts of the image, the image background may become too dark.